Inkjet printing is a versatile and widely used form of print imaging. So called ‘drop-on-demand’ inkjet printing (as opposed to continuous inkjet printing) is the ejection of ink drops by forming vapor bubbles in a bubble forming liquid. This principle is generally described in U.S. Pat. No. 3,747,120 (Stemme). Each pixel in the printed image is derived ink drops ejected from one or more ink nozzles. Many different aspects and techniques for inkjet printing are described in detail in the above cross referenced documents.
Inkjet printers have a printhead with an array of nozzles though which ink is ejected onto a media substrate such as paper or film. Typical SOHO (Small Office, Home Office) inkjet printers or wide format inkjet printers have a scanning printhead. The printhead scans across the printed width of the media substrate and prints a swathe of the printed image with each traverse.
More recently, pagewidth printers have been developed to speed up the printing process. A pagewidth printhead remains stationary within the printer and has an array of nozzles that extends the entire printing width of the media substrate. Media substrate passes through the printer as the printhead prints the width of the media simultaneously. By not traversing the printhead across the media as it indexes through the printer, print speeds are significantly increased.
The gap between the nozzle array and the surface of the media substrate is referred to as the ‘print gap’ or the printhead to paper separation (PPS). This gap is typically less than 3 mm. However, the movement of the media substrate and the ejection of ink drops can generate vortices in the air flow through the print gap. Under certain conditions, the vortices in the air flow oscillate and skew the trajectories of the ejected ink drops. This produces visible artifacts in the printed image and degrades print quality. The artifacts appear as a series of irregular bands extending generally transverse to the media feed direction and are generally referred to as ‘tiger stripes’, ‘sand dunes’, ‘wood grain’ or ‘worms’.